NAS 125: Meteorology

Humidity, Saturation, and Stability

Rev. 2 March 2006 Humidity, Saturation, and Stability 2

Cloud forests, part 1

• Some tropical and subtropical forests are perpetually enshrouded in clouds and mist; these are called cloud forests.

• Cloud forests play an important role in terms of biodiversity and water supply, but they are among the most threatened ecosystems in the world as a result of development pressures as well as possible climate change.– There are 605 cloud forests in 41 nations, primarily in

Central and South America.

Rev. 2 March 2006 Humidity, Saturation, and Stability 3

Rev. 2 March 2006 Humidity, Saturation, and Stability 4

Rev. 2 March 2006 Humidity, Saturation, and Stability 5

Cloud forests, part 2

• Cloud forests are typically found at elevations ranging from 2,000 m to 3,500 m, but may be found at lower elevation on humid islands.

• Onshore, upslope flow of warm, moist air helps fuels the low clouds, fog, and mist characteristic of cloud forests.– Air becomes saturated with moisture as it rises and cools.

– Forest canopy traps the moisture, encouraging the formation of water droplets that drip to the forest floor.

• Trapped moisture is equivalent to 20 percent to 60 percent of the local precipitation totals.

Rev. 2 March 2006 Humidity, Saturation, and Stability 6

Cloud forests, part 3

• Persistent cloud cover reinforces the cool, moist conditions by reducing solar radiation reaching the ground and in turn suppressing the evapotranspiration of water from the surface.

• Climate change could reduce the extent of cloud forests by increasing the amount of cooling necessary to produce conditions favorable for condensation of water, but could conversely increase the amount of water vapor in the air that feeds the cloud forest ecosystems.

Rev. 2 March 2006 Humidity, Saturation, and Stability 7

Water

• Water occurs in all three states of matter:– Solid (snow, sleet, hail, ice);

– Liquid (rain, water droplets);

– and Gas (water vapor).

• The gaseous state is the most important driver of the dynamics of the atmosphere.– Changes of state of water serve an important role in

transferring energy through the Earth-atmosphere system.

Rev. 2 March 2006 Humidity, Saturation, and Stability 8

Reservoirs of water

• The primary reservoir of water is the ocean basins (97 percent).

• Two percent of water is in ice sheets and glaciers.• About 0.6 percent is in groundwater.• Very little water is stored in the atmosphere.

– But the atmosphere is the primary conduit for bringing water from the oceans to the land surfaces.

Rev. 2 March 2006 Humidity, Saturation, and Stability 9

Rev. 2 March 2006 Humidity, Saturation, and Stability 10

Hydrologic cycle, part 1

• Water is distributed very unevenly around Earth.• Less than 1% of Earth’s total moisture is involved in

the hydrologic cycle.• The hydrologic cycle is a series of storage areas

interconnected by various transfer processes, in which there is a ceaseless interchange of moisture in terms of its geographical location and its physical state.

Rev. 2 March 2006 Humidity, Saturation, and Stability 11

Hydrologic cycle, part 2

• Surface-to-air water movement– Evaporation is responsible for most of the moisture that

enters the atmosphere from Earth’s surface.• Of the moisture evaporated, more than 84% comes from ocean

surfaces.

• The water evaporated becomes water vapor, and though it stays in atmosphere only briefly (hours to days), it can travel a considerable distance, either vertically or horizontally.

Rev. 2 March 2006 Humidity, Saturation, and Stability 12

Hydrologic cycle, part 3

• Air-to-surface water movement– Water vapor will either condense to liquid water or

sublimate to ice to form cloud particles.

– Clouds drop precipitation (rain, snow, sleet, hail).

– Precipitation and evaporation/transpiration balance in time.• They do not balance in place.

• Evaporation exceeds precipitation over oceans.

• Precipitation exceeds evaporation over lands.

Rev. 2 March 2006 Humidity, Saturation, and Stability 13

Hydrologic cycle, part 4

• Movement on and beneath Earth’s surface– Runoff is the flow of water from land to oceans by

overland flow, streamflow, and groundwater flow.• Runoff is why the oceans do not dry up and continents become

flooded despite the imbalance of evaporation and precipitation through space (oceans and continents).

• Runoff water amounts to 8% of all moisture circulating in global hydrologic cycle.

Rev. 2 March 2006 Humidity, Saturation, and Stability 14

Hydrologic cycle, part 5

• Residence times– At any given movement, the atmosphere contains only a

few days’ potential precipitation.

– The residence time of a molecule of water can be hundreds of thousands of years to only a few minutes.

Rev. 2 March 2006 Humidity, Saturation, and Stability 15

Rev. 2 March 2006 Humidity, Saturation, and Stability 16

Rev. 2 March 2006 Humidity, Saturation, and Stability 17

Evaporation, part 1

• Evaporation is the process by which liquid water is converted to gaseous water vapor.– Molecules of water escape the liquid surface into the

surrounding air.

• Temperature is a key factor in evaporation, both in water and in the air around it.– Molecules become more agitated the higher the

temperature, and this agitation leads to evaporation.

Rev. 2 March 2006 Humidity, Saturation, and Stability 18

Evaporation, part 2

• Temperature works in conjunction with pressure.– Vapor pressure is the pressure exerted by water vapor in

the air. • At any given temperature, there is a maximum vapor pressure that

water vapor molecules can exert.

• Saturated air: Air becomes saturated when it reaches the point at which some water vapor molecules must become liquid because maximum vapor pressure is exceeded.

• The warmer the air, the more water vapor it can hold before becoming saturated.

Rev. 2 March 2006 Humidity, Saturation, and Stability 19

Evaporation, part 3

• Still Versus Moving Air– Movement in air through windiness and/or turbulence helps

promote evaporation by removing saturated air.• Disperses vapor molecules and thus makes air above water surface

less saturated, so rate of evaporation can increase.

Rev. 2 March 2006 Humidity, Saturation, and Stability 20

Evapotranspiration, part 1

• Evapotranspiration is the process of water vapor entering the air from land sources.– Evapotranspiration occurs through two ways:

• Transpiration is the process by which plant leaves give up their moisture to the atmosphere;

• Evaporation from soil and plants.

– Most evapotranspiration occurs through plants.

Rev. 2 March 2006 Humidity, Saturation, and Stability 21

Evapotranspiration, part 2

• Potential evapotranspiration is the maximum amount of moisture that could be lost from soil and vegetation if the ground were sopping wet all the time.

• Potential evapotranspiration rate and actual rate of precipitation play a key role in determining a region’s groundwater supply (or lack of it).

Rev. 2 March 2006 Humidity, Saturation, and Stability 22

Condensation, part 1

• Condensation is the process by which water vapor is converted to liquid water; it is the opposite of evaporation.

• For condensation to take place, air must be saturated.– Condensation cannot occur, however, even if the air is

saturated, if there is not a surface on which it can take place.

• Air becomes supersaturated if surface is not available.

Rev. 2 March 2006 Humidity, Saturation, and Stability 23

Condensation, part 2

• Air must be saturated (continued):– In upper atmosphere, surfaces are available through

hygroscopic particles or condensation nuclei—tiny atmospheric particles of dust, smoke, and salt that serve as collection centers for water molecules.

• Most common are bacteria blown off plants or thrown into air by ocean waves.

Rev. 2 March 2006 Humidity, Saturation, and Stability 24

Other state changes

• Freezing is the process by which liquid water is converted to ice, thus giving off heat.

• Melting is the process by which ice is converted to liquid water, thus absorbing heat.

• Sublimation is the process by which water vapor is converted directly to ice, or vice versa.

Rev. 2 March 2006 Humidity, Saturation, and Stability 25

Rev. 2 March 2006 Humidity, Saturation, and Stability 26

Global water budget

• The amount of water falling on land surfaces exceeds the amount lost from the land surfaces to the air via evapotranspiration or sublimation.

• The amount of water evaporating from oceans exceeds that falling over the oceans.

• Excess water from land returns to oceans via runoff.

Rev. 2 March 2006 Humidity, Saturation, and Stability 27

Rev. 2 March 2006 Humidity, Saturation, and Stability 28

Rev. 2 March 2006 Humidity, Saturation, and Stability 29

Measures of humidity, part 1

• Humidity is the amount of water vapor in the air.• Dalton’s law: The total pressure of a mixture of gases

equals the sum of the partial pressures contributed by each gas.– The partial pressure contributed by water vapor is called

the vapor pressure.

• Air is said to be saturated if it is at its maximum humidity – when the amount of water molecules leaving the liquid state to become a gas equals the number leaving the gaseous state to become liquid.

Rev. 2 March 2006 Humidity, Saturation, and Stability 30

Measures of humidity, part 2

• Mixing ratio is the ratio of the mass of water vapor per mass of the remaining dry air.

• Absolute humidity is a direct measure of the water vapor content of air.– It is expressed as the weight of water vapor in a given

volume of air, usually as grams of water per cubic meter of air.

• The amount is a function of how much volume is being considered.– If the volume of air doubles, the absolute humidity halves.

• Absolute humidity is limited according to temperature.– The colder the air, the less vapor it can hold.

Rev. 2 March 2006 Humidity, Saturation, and Stability 31

Measures of humidity, part 3

• Specific humidity is a direct measure of water-vapor content expressed as the mass of water vapor in a given mass of air (grams of vapor/kilograms of air).

• The saturation vapor pressure of air is a function of the temperature of air – the amount of water air can hold increases with increasing temperature.– Related measures:

• Saturation mixing ratio

• Saturation absolute humidity

• Saturation specific humidity

Rev. 2 March 2006 Humidity, Saturation, and Stability 32

Rev. 2 March 2006 Humidity, Saturation, and Stability 33

Rev. 2 March 2006 Humidity, Saturation, and Stability 34

Rev. 2 March 2006 Humidity, Saturation, and Stability 35

Measures of humidity, part 4

• Relative humidity is an expression of the amount of water vapor in the air in comparison with the total amount that could be there if the air were saturated. This is a ratio that is expressed as a percentage.– Relative Humidity = Actual Water Vapor in Air/Capacity x

100

– Relative humidity changes if either the water vapor content or the water vapor capacity of the air changes.

– Calculation of relative humidity:• RH = (vapor pressure/saturation vapor pressure) * 100

• RH = (mixing ratio/saturation mixing ratio) * 100

Rev. 2 March 2006 Humidity, Saturation, and Stability 36

Rev. 2 March 2006 Humidity, Saturation, and Stability 37

Measures of humidity, part 5

• Relative humidity (continued):– Relative humidity also changes if temperature changes.

• The relationship between temperature and relative humidity is one of most important in all meteorology.

• Relative humidity and temperature have an inverse relationship – as one increases, the other decreases.

• Relative humidity can be determined through the use of a psychrometer.

Rev. 2 March 2006 Humidity, Saturation, and Stability 38

Rev. 2 March 2006 Humidity, Saturation, and Stability 39

Measures of humidity, part 6

• Cooling is the most common way that air is brought to the point of saturation and condensation.– Dew point is the temperature to which air must be cooled

to achieve saturation.• Dew refers to water droplets that form when water vapor

condenses on a cold surface.

– Frost point is the air temperature at which frost forms.• Frost refers to ice crystals that form when air becomes saturated at

a temperature below freezing.

Rev. 2 March 2006 Humidity, Saturation, and Stability 40

Measures of humidity, part 7

• Precipitable water is the depth of liquid water that would be formed if all water vapor in a column of air condenses.– All water in the atmosphere would reach 2.5 cm

– Precipitable water ranges from more than 4.0 cm in the humid tropics to 0.5 cm in polar regions.

Rev. 2 March 2006 Humidity, Saturation, and Stability 41

Rev. 2 March 2006 Humidity, Saturation, and Stability 42

Measurement instruments, part 1

• A hygrometer measures the water vapor concentration of air.– A dewpoint hygrometer air passes over the surface of a

metallic mirror that is cooled electronically; the temperature at which a condensation film forms on the mirror is recorded as the dewpoint temperature.

– A hair hygrometer uses human hair, which lengthens as it absorbs water and shortens as it dries out; hair is connected to a dial calibrated to read in percent relative humidity.

• A hydrograph uses a hair hydrograph and it traces the trend in relative humidity on paper pulled across a clock-driven drum.

Rev. 2 March 2006 Humidity, Saturation, and Stability 43

Measurement instruments, part 2

• Hygrometer (continued):– Electronic hygrometers measure the change in electrical

resistance of certain materials as they respond to changing relative humidity.

• Psychrometers measures temperature with two identical liquid-in-glass thermometers.– One has a dry bulb; it measures actual air temperature (dry-

bulb temperature).

– One has a bulb wrapped in muslin, that is in turn moistened; it measures the wet-bulb temperature.

Rev. 2 March 2006 Humidity, Saturation, and Stability 44

Measurement instruments, part 3

• Psychrometers (continued):– The psychrometer can be either whirled around by hand (a

sling psychrometer) or it can be ventilated by a small fan (an aspirated psychrometer).

– The drier the air, the greater the evaporation from the wet bulb thermometer, thus the cooler the wet-bulb temperature.

• The difference between the air temperature and the wet-bulb temperature is called the wet-bulb depression.

– The relative humidity is looked up on a chart using the dry-bulb temperature and wet-bulb depression.

Rev. 2 March 2006 Humidity, Saturation, and Stability 45

Rev. 2 March 2006 Humidity, Saturation, and Stability 46

Rev. 2 March 2006 Humidity, Saturation, and Stability 47

Measurement instruments, part 4

• Psychrometers (continued):– It is difficult to get accurate wet-bulb temperatures under

cold or subfreezing conditions as well as under very dry conditions.

– The dry-bulb temperature and wet-bulb temperature can be used to look up the dewpoint temperature.

Rev. 2 March 2006 Humidity, Saturation, and Stability 48

Rev. 2 March 2006 Humidity, Saturation, and Stability 49

Water vapor satellites

• Water vapor satellite images use special infrared sensors to measure water vapor in the atmosphere.– Water vapor does not appear on visible or on conventional

infrared sensors.

– Such images allow meteorologists to track movement of plumes of moisture through the atmosphere.

– Current platforms measure clouds and water vapor concentrations above 3,000 m.

– A gray scale is used, so that little or no water vapor appears black, while high concentrations appear milky white. Upper-level clouds appear as bright white blotches.

Rev. 2 March 2006 Humidity, Saturation, and Stability 50

Rev. 2 March 2006 Humidity, Saturation, and Stability 51

Rev. 2 March 2006 Humidity, Saturation, and Stability 52

Volume and temperature

• The principal way that clouds form in the atmosphere is by expansional cooling.– When a parcel of warm air rises, the pressure upon it is

less, so the parcel expands; according to the ideal gas law, temperature of a gas decreases as the volume increases.

• Conversely, descending air dries out by compressional warming.– When a parcel of air descends, the pressure upon it

increases, so the parcel contracts; according to the ideal gas law, the temperature of a gas increases as the volume decreases.

Rev. 2 March 2006 Humidity, Saturation, and Stability 53

Adiabatic processes, part 1

• Adiabatic cooling or warming of parcels of air occurs without any heat exchanged between the parcel and the surrounding environment.

• The dry adiabatic lapse rate is the rate at which a parcel of unsaturated air cools as it rises; this rate is relatively steady (10 °C/km). – Air is not necessarily “dry,” just not saturated.

– Descending air warms, and it does so at the dry adiabatic lapse rate.

Rev. 2 March 2006 Humidity, Saturation, and Stability 54

Rev. 2 March 2006 Humidity, Saturation, and Stability 55

Adiabatic processes, part 2

• Lifting condensation level (LCL) is the altitude at which rising air cools. sufficiently to reach 100% relative humidity at the dew point temperature, and condensation begins.

• The saturated adiabatic lapse rate is the diminished rate of cooling, which occurs when air rises above the lifting condensation level. It depends on temperature and pressure, but averages about 6 °C/km.

Rev. 2 March 2006 Humidity, Saturation, and Stability 56

Rev. 2 March 2006 Humidity, Saturation, and Stability 57

Rev. 2 March 2006 Humidity, Saturation, and Stability 58

Rev. 2 March 2006 Humidity, Saturation, and Stability 59

Buoyancy of air, part 1

• Buoyancy is the tendency of an object to rise in a fluid.– A parcel of air moves vertically until it reaches a level at

which the surrounding air is of equal density (equilibrium level).

• Stability– Stable air resists vertical movement; it is nonbuoyant, so it

will not move unless force is applied.

– Unstable air is buoyant; it will rise without external force or will continue to rise after force is removed.

Rev. 2 March 2006 Humidity, Saturation, and Stability 60

Buoyancy of air, part 2

• Stability (continued):– Conditional instability is intermediate condition between

absolute stability and absolute instability; it occurs when an air parcel’s adiabatic lapse rate is somewhere between the dry and wet adiabatic rates. Conditionally unstable air acts like stable air until an external force is applied; when forced to rise, it may become unstable if condensation occurs (which releases latent heat that provides buoyancy).

Rev. 2 March 2006 Humidity, Saturation, and Stability 61

Buoyancy of air, part 3

• Determining air stability– Air stability is related to adiabatic temperature changes.

– Accurate determination of stability of any mass of air depends on temperature measurements, but one can get a rough indication from looking at cloud patterns.

• Unstable air is associated with distinct updrafts, which are likely to produce vertical clouds.

• Cumulous clouds suggest instability.

• Towering cumulonimbus clouds suggest pronounced instability.

Rev. 2 March 2006 Humidity, Saturation, and Stability 62

Buoyancy of air, part 4

• Determining air stability (continued):– Accurate determination of stability (continued):

• Horizontally developed clouds, most notably stratiform, characterize stable air forced to rise.

• Cloudless sky indicative of stable, immobile air.

– Soundings (temperture profiles) of the atmosphere are useful tools in determination of the stability of air.

Rev. 2 March 2006 Humidity, Saturation, and Stability 63

Rev. 2 March 2006 Humidity, Saturation, and Stability 64

Stüve diagrams, part 1

• Soundings and adiabatic processes can be illustrated graphically with Stüve diagrams.

• Characteristics of Stüve diagrams– X-axis: temperature (increasing to right)– Y-axis: pressure (decreasing upward)

• Also altitude, based on standard atmosphere

– Dry adiabatic lapse rates denoted by straight red lines (dry adiabats) trending from lower right to upper left

– Wet adiabatic lapse rates denoted by curved dashed lines (wet adiabats)

– Sauration mixing ratio (slanted black lines; g/kg)

Rev. 2 March 2006 Humidity, Saturation, and Stability 65

Rev. 2 March 2006 Humidity, Saturation, and Stability 66

Stüve diagrams, part 2

• Interpretation of Stüve diagrams– Unsaturated air follows dry adiabats, or a line that parallels

the dry adiabats.

– Saturated air follows moist adiabats or a curve that more or less parallels the moist adiabats.

• Moist adiabats become nearly indistinguishable from dry adiabats at very low pressures and temperatures.

– Plots of soundings on a Stüve diagram enables a meteorologist to assess atmospheric stability and the potential for convective activity.

Rev. 2 March 2006 Humidity, Saturation, and Stability 67

Stüve diagrams, part 3

• Interpretation of Stüve diagrams (continued):– Plotted values of the saturation mixing ratio helps

meteorologists determine relative humidity and saturation levels of rising air parcels.

Rev. 2 March 2006 Humidity, Saturation, and Stability 68

Rev. 2 March 2006 Humidity, Saturation, and Stability 69

Rev. 2 March 2006 Humidity, Saturation, and Stability 70

Convection currents

• Heated air expands and moves upward in the direction of lowest pressure. The cooler surrounding air then moves in to fill the empty space, and the air from above moves in to replace that cooler air. One ends up with an updraft of warm air and a downdraft of cool air. – Convection currents fuel storm development.

• A similar convective system occurs in each hemisphere during its summer and throughout the year in the tropics.

Rev. 2 March 2006 Humidity, Saturation, and Stability 71

Rev. 2 March 2006 Humidity, Saturation, and Stability 72

Rev. 2 March 2006 Humidity, Saturation, and Stability 73

Lifting and precipitation, part 1

• Significant amounts of precipitation can originate only by rising air and adiabatic cooling.

• There are four principal types of atmospheric lifting:– Convective lifting

– Orographic lifting

– Frontal lifting

– Convergent lifting

• More often than not, the various types operate in conjunction with one another.

Rev. 2 March 2006 Humidity, Saturation, and Stability 74

Lifting and precipitation, part 2

• Convective precipitation is showery precipitation with large raindrops falling fast and hard; caused by convective lifting, which occurs when unequal heating of different air surface areas warms one parcel of air and not the air around it.– This is the only spontaneous of the four lifting types; the

other three require an external force.

Rev. 2 March 2006 Humidity, Saturation, and Stability 75

Lifting and precipitation, part 3

• Orographic precipitation occurs with orographic lifting, caused when topographic barriers force air to ascend upslope; only occurs if the ascending air is cooled to the dew point.– A rain shadow is an area of low rainfall on the leeward side

of a topographic barrier; can also apply to the area beyond the leeward side, for as long as the drying influence continues.

Rev. 2 March 2006 Humidity, Saturation, and Stability 76

Rev. 2 March 2006 Humidity, Saturation, and Stability 77

Lifting and precipitation, part 4

• Frontal precipitation occurs when air is cooled to the dew point after unlike air masses meet, creating a zone of discontinuity (front) that forces the warmer air to rise over the cooler air (frontal lifting).

• Convergent precipitation is showery precipitation caused by convergent lifting, the least common form of lifting, which occurs when air parcels converge and the crowding forces uplift, which enhances instability. This precipitation is particularly characteristic of low latitudes